It has been known that there is a system for driving an alternate current motor in which the whole speed range of the motor is divided into the following two ranges. That is, the A-range which is the range in which the speed of the motor is lower than the base speed and the motor is driven in accordance with the constant-torque characteristic and the B-range which is the range in which the speed of the motor is higher than the base speed and the motor is driven in accordance with the constant-power characteristic (FIG. 4). In the prior art, a system for driving a motor by a PWM inverter in the A- and B-range has been proposed. However, in this prior art, the torque of the motor in the A-range is not sufficient because the output voltage of the PWM inverter in the A-range is too low, and accordingly, it is difficult to realize a smooth running of the motor in the A-range. In addition, the transition of the running of the motor between the A- and B-ranges is not effected smoothly.
The present invention has been proposed in order to solve these problems in the system for driving a motor by a pulse width modulation (PWM) inverter for the A- and B-ranges. The above mentioned prior art is disclosed in, for example, the catalogue "Solid State Adjustable Frequency Drives TR 1000 and TR 3000 Series" issued by the PTI Controls Co. and the catalogue "Electronic Variable Frequency Drive-VFD Series" issued by the MIKI Pulley Co.